its almost impossible to avoid some silly mistakes, this is because whether how much u study and revise for any exams, u will be nervious, which is just enough for anyone to lose a few marks due to the lack of confidence.
but the best way how to avoid it is try to feel as much confident as possible, meaning study on easy and hard stuff, esp on harder topics and doing as much different types of calculations as possible, it might be enough to save u 10 marks ?
its also veryyy important to try and regain confidence by forgetting about other stuff. Like while studying u feel hungry or need to go to the toilet, ur brain puts those stuff as 2nd priority, and studying as 1st priority. It makes the brain think more about other stuff and cause u to focus less on ur revison. In this case go eat or go to the toilet, after ur done u should have more time to focus which motivateu to study
Hope this helps